A microwave filter is an electromagnetic circuit that can be tuned to pass energy at a specified resonant frequency. Accordingly, microwave filters are commonly used in telecommunication applications to transmit energy in a desired band of frequencies (i.e. the passband) and reject energy at unwanted frequencies (i.e. the stopband) that are outside the desired band. In addition, the microwave filter should preferably meet some performance criteria for properties, which typically include insertion loss (i.e. the minimum loss in the passband), loss variation (i.e. the flatness of the insertion loss in the passband), rejection or isolation (the attenuation in the stopband), group delay (i.e. related to the phase characteristics of the filter) and return loss.
A group of microwave filters developed during and since World War II are generally known as waveguide or cavity filters. These filters are hollow structures of different shapes and are sized to resonate at specific frequency bandwidths in response to microwave signals. A common waveguide filter 2 having a plurality of waveguide resonators is shown in FIG. 1A. The walls formed between each pair of adjacent resonator cavities 1 are provided with an iris 3. Each iris 3 provides a means for the near-lossless or conventional coupling of electromagnetic energy between adjacent waveguide resonators. Resonant energy will collect and flow through each waveguide resonator as the signal passes through the waveguide filter 2. The performance may be improved and the cavity size reduced by inserting materials into the resonators.
Referring now to the dielectric filter assembly 4 of FIG. 1B, low-loss dielectric resonators 6 are commonly used to improve performance. Common implementations of a dielectric resonator 6 include positioning a dielectric puck 6a on a pedestal 6b within the resonator cavity 1. Filters incorporating dielectric resonator assemblies can have quality factors (Q factors) in the range of 8000 to 15,000.
Similarly, in a combline filter assembly 7, as shown in FIG. 1C, metal combline resonators 8 are positioned within the resonator assemblies 1. The combline resonator 8 is normally housed within and is in electrical contact at one end with the metallic cavity 1. Although resulting in a much lower Q factor, combline filter assemblies 7 normally benefit from a reduction in cavity filter size and excellent spurious performance. Under comparable design criteria, a combline filter is approximately half of the size of a dielectric cavity filter but has about half the Q factor.
The size of the cavity and the materials chosen determine the Q factor for a resonator. The Q factor compares the resonant frequency of a system to the rate at which it dissipates its energy. The Q factor of the individual resonators has a direct effect on the amount of insertion loss and pass-band flatness of the realized microwave filter. In particular, a resonator having a higher Q factor will have lower insertion loss and sharper slopes. This results in frequency response that is idealized as a block filter with a flat passband and sharp slopes at the cutoff frequencies. In contrast, filters that have a low Q factor have a larger amount of energy dissipation due to larger insertion loss and will also exhibit a larger degradation in band edge sharpness resulting in a more rounded response.
The comparison in frequency responses 9 in FIG. 2 highlights the effects of an unloaded Q factor on the frequency response of a filter. The frequency response of Q1 shows rounded band edges when the Q factor is 100. High Q factors result in better filter performance as shown in Q3 and Q4.
Filter design is usually a trade off between all of the in-band and out-of-band parameters. A transfer function is a well-known approach to expressing the functionality of a microwave filter in polynomial form. Once a desired transfer function for a desired filter is created, the material type and size of resonators are chosen. The types of resonators used limit the Q factor. In order to increase the Q factor, one often has to increase the size of the resonators resulting in a larger and heavier filter. This is disadvantageous since multi-cavity microwave filters are typically used in various space craft communication systems such as communication satellites in which there are stringent restrictions on payload mass. The finite Q factor (highest possible value selected after the trade off between size and performance is made) will translate to energy dissipation and non-idealized performance. Accordingly, the transfer function of the realized microwave filter will have passband edges that slump downward which causes unwanted distortion and intermodulation.
In order to improve the filter parameters such as loss variation, (band edge sharpness) without resorting to an increase in size and mass, a number of techniques have been discovered. The concept of adaptive predistortion is disclosed by Yu in U.S. Pat. No. 6,882,251 which describes the use of return loss distortion to equalize the transmission response, essentially bouncing back more energy at the band centre to equalize the response in the passband. This method for cross-coupled microwave filters results in an improved filter response in the passband but very poor return loss responses (3-6 dB typical).
Another technique uses resonators with non-uniform Q factors to create non-uniform dissipation in the resonator network. The design by Guyette, Hunger, and Pollard, entitled, “The Design of Microwave Bandpass Filters Using Resonators with Nonuniform Q,” describes a method of combining low Q factor resonator paths on the outsides of a multi-resonator microstrip filter to improve the full response when the paths are combined. With this configuration, the multiple signal paths form the full response in a manner similar to active channelized filters. One path forms the response at the band edges, while another path forms the response at the centre of the passband. The full response of the two paths creates a microstrip filter with high selectivity at the expense of increased insertion loss for a given average Q factor.